1. Technical Field
The present invention relates in general to an improved voice coil motor and, in particular, to an improved system, method, and apparatus for plastically deforming and potting the voice coil of a voice coil motor for a disk drive.
2. Description of the Related Art
Data access and storage systems generally comprise one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, one to five disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm). Hard disk drives have several different typical standard sizes or formats, including server, desktop, mobile (2.5 and 1.8 inches) and microdrive.
A typical HDD also uses an actuator assembly to move magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
A slider is typically formed with an aerodynamic pattern of protrusions on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. A slider is associated with the functional side of each disk and flies just over the disk's surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to a semi-rigid actuator arm that supports the entire head flying unit. Several semi-rigid arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track.
The motor used to rotate the disk is typically a brushless DC motor. The disk is mounted and clamped to a hub of the motor. The hub provides a disk mounting surface and a means to attach an additional part or parts to clamp the disk to the hub. In most typical motor configurations of HDDs, the rotating part of the motor (the rotor) is attached to or is an integral part of the hub. The rotor includes a ring-shaped magnet with alternating north/south poles arranged radially and a ferrous metal backing. The magnet interacts with the motor's stator by means of magnetic forces. Magnetic fields and resulting magnetic forces are induced via the electric current in the coiled wire of the motor stator. The ferrous metal backing of the rotor acts as a magnetic return path.
In prior art carriage and head stack assembly (HSA) performance, it has long been desired to stiffen the coil to improve its dynamic performance. Attempts at doing this have taken two directions. First, the coil has been cast in a plastic carrier or yoke and attached to the barrel of the comb. Drawbacks to this approach are that plastic is not the best structural material for drives requiring high dynamic performance, nor is it an effective conductor of heat for cooling the coil. This approach has found limited acceptance in the mobile drive market. Another approach has been to add a bobbin to the center of the coil. While increasing stiffness, it also increases mass, which detracts from power consumption and/or access time.
Another problem is that the coil generates heat and can reach 100° C. in server class drives and requires measures for cooling in the design of the HSA. The current method of mounting the coils for cooling is to bond the coil to the yoke with an adhesive that is filled with aluminum particulate to increase its thermal conductivity. There are a few problems with this approach. The width of the adhesive bond (about 0.5 mm) has relatively low modulus and stiffness as compared to the aluminum of the yoke. Variability in its width as well as variability in its properties, such as glass transition temperature (Tg), can cause variability in the dynamic performance of the HSA. In addition, variability in the homogeneity as well as the amount of aluminum particle filler will cause variability in the adhesives thermal conductivity and, consequently, variability in the power consumption or access speed of the HSA.
Moreover, the amount of adhesive required to bond the coil to the yoke is denser than an equivalent volume of coil windings. For a given carriage, this equates to about 0.2 g. Added mass either reduces the access time or increases power consumption. Furthermore, the aforementioned 0.5 mm bond line in effect positions the yoke legs outwards by this amount. This has an adverse effect on coil torsion modes.
Finally, the coil location in the z-axis of the carriage as well as planarity of the coil to the VCM magnets has associated tolerances. These tolerances exist with all coil attachment processes and can only be improved with better tooling and bonding fixtures. It is suspected that the coil not being centered or out of plane with the VCM creates coil torsion problems. Thus, an improved VCM assembly solution would be desirable.